In mobile communication systems, such as cellular radio systems or trunking radio systems, mobile radio stations may move freely from one radio cell to another. When leaving a previous radio cell, a mobile station has to search for another cell and to register to this new cell. In order to be able to make handover from one cell to another during an on-going call without significant break in the call, the mobile station should be prepared for a change of cell preferably all the time, by monitoring the carriers of the base stations of adjacent cells. Thus there is continuously available precollected information on the coverage of the adjacent cells and the mobile station or the fixed network is capable of making quick decision to choose the best adjacent cell for a handover, when the carrier of the present cell becomes weaker. Thus a handover may be performed for instance during a call without noticeable break in the call.
In radio systems of TDMA type, a plurality of physical channels, i.e. time slots, are arranged for each carrier of a base station by time division multiplexing. In FIG. 1 for example, four time slots 0, 1, 2 and 3 are repeated on the carriers of base stations BS1 and BS2. On these physical channels are conveyed logical channels, which may be divided into two categories: traffic channels transmitting user information and control channels transmitting signalling messages. When a mobile station MS is registered to a base station BS and monitors the carrier thereof, it is normally locked to one of the TDMA time slots, e.g. the time slot 0 of a base station BS1 in FIG. 1. The other time slots 1, 2 and 3 of BS1 are then of no significance for the MS and therefore, the MS may measure the carriers of the adjacent base stations during these time slots. If only measurement of the intensity of a received signal is required and no decoding of the time slot of the adjacent carrier is necessary, no problems will appear with finding the correct time slot. Still, the switching time (the time required for tuning from one frequency to another) of a synthetizer of the MS must be short enough. However, if it is necessary, besides measuring the intensity of the received signal, to decode the information contained in the time slot, in order to derive, e.g., base station identification, system identity, traffic load level or transmission power level, timing problems arise, which are caused by the limited switching time of the MS synthetizer. In the case of FIG. 1, for instance, due to the abovementioned timing problems, the mobile station has time to measure and decode, between the time slots 0 of the base station BS1, only that time slot of the carrier of a base station BS2 which is located approximately at the time slot 2. We may presume that the base station BS2 and the other base stations have the main control channel, which should be measured, in the TDMA time slot 0. Thus the MS may hardly ever be capable of measuring the correct time slot.
If the MS monitors the main control channel (time slot 0) of the base station BS1, it is possible to take the risk and skip over this time slot occasionally and to decode the main control time slot 0 of the adjacent base station. A drawback will be a longer and less reliable call establishment, because the first paging message addressed to the MS may be lost in between. If the MS monitors a traffic channel time slot, it is possible to interrupt reception and speech coding and to skip over this time slot occasionally for decoding the main control channel time slot of the adjacent base station. Drawbacks of this approach will be a lower speech quality and a possibility of the message becoming incomprehensible.